Abstraction - Prevent developer from providing incomplete functions

• Programmatically guide pyexcel extension developers on what methods should have been implemented.
  • It wont allow you to instantiate from you custom class unless you implement all abstract methods
• Passing custom keywords to your classes
• Providing NamedContent as a helper class for data content that does not have a name but a name(sheet name) is required when writing a pyexcel extension.